Fine-scale zooplankton diel vertical migration revealed by traditional net sampling and a Laser Optical Plankton Counter (LOPC) in Lake Ontario

Traditional net hauls used to monitor zooplankton population change in the Great Lakes do not provide information on vertical distribution - an important aspect of the availability of zooplankton to predators. During 2013, we combined net data with data from a Laser Optical Plankton Counter (LOPC) to track the vertical distribution of zooplankton in Lake Ontario in May, July, and September. Closing nets (64 μm mesh) sampled three strata at stations of north-south transects. For a 10 km segment of each transect in May and September, a LOPC was towed from the surface to 60 m depth. In May, biomass was highest in the epilimnion both day and night. Diacyclops thomasi dominated in the epilimnion while Limnocalanus macrurus dominated deeper. During strongly stratified conditions in July and September, closing net data confirmed that biomass was highest in the metalimnion during the day and in the epilimnion during the night. D. thomasi, Daphnia retrocurva, and L. macrurus were major components and vertical migrators. In May, LOPC data showed that biomass was highest in the upper 10 m of the epilimnion for day and night. In September, LOPC data showed two layers during the day, a narrow 5 m layer in the epilimnion at about 20 °C and a 15 m layer below the thermocline at 10 °C. The lower layer followed the deepening thermocline from west to east. At night, the LOPC detected most biomass throughout the epilimnion. The LOPC data confirmed diel vertical migration and detailed the fine-scale structure of zooplankton layers.